Fiber optics deals with the transmission of light through small filamentary optical materials or fibers. The fibers comprise a central core and an outer surrounding cladding along the entire length of the fiber. The transmission of light through the fiber is based on the phenomenon of total internal reflection. For total internal reflection, the refractive index (n.sub.D) of the core must be greater than the refractive index of the cladding.
Depending on the application, the materials used to fabricate the optical fiber vary. For example, long-haul or telecommunications-based optical fibers must provide low transmission loss and low absorption coefficient. Thus, silica-based materials are predominantly used to construct the fiber. On the other hand, short-haul applications such as optical fibers used in medical surgery instruments need not provide such a low transmission loss and the absorption coefficient is not as critical. Instead of specifically tailoring the materials to satisfy the needs of the medical community, the fibers used for surgical laser applications are still based on materials and properties that are more suitable for long-haul applications.
One type of short-haul application is the use of optical fibers for the transmission of mid-infrared (approximately 2-6 .mu.m) laser light in medical instruments designed for invasive surgery. For such an application, the glass used to fabricate the fiber must satisfy other criteria such as non-toxicity, high laser damage threshold, excellent visible transmission for using an aim beam to direct the laser beam, low absorption coefficient at the laser wavelength, high glass transition temperature (T.sub.g), low expansion coefficient (.alpha.) high softening temperature (T.sub.g) high chemical durability in water characterized by low values of D.sub.W, low liquidus temperature (T.sub.l) small temperature dependence of viscosity in the fiber drawing temperature region, and a strong resistance to devitrification. Additionally, mechanical and power handling properties are important for some applications. No glass composition for optical fibers in the present state of the art meets all of these criteria.
When these medical instruments are used to cut tissue with laser light, the laser light must be highly absorbed by the tissue. Because water is a ubiquitous and large constituent of animal tissue, medical researchers have naturally focused their attention on developing lasers which operate at wavelengths of light which are strongly absorbed by water. High absorption implies that the laser light is absorbed by water in the tissue completely before it travels (or penetrates) into the tissue.
The highest water absorption occurs near 3 .mu.m. Pulsed Cr:Tm:Er:YAG, Er:YLF, Er:YSGG, and Er:YAG lasers operating at 2.69, 2.71, 2.796, and 2.94 .mu.m are of great interest for medical applications because their radiation is not only highly absorbed by water but also by the organic matrix and inorganic calcium salts which comprise bone, enamel, and other biological materials. This very superficial penetration depth allows both hard and soft tissues to be ablated very precisely and quickly with virtually no tissue charring.
One reliable and well developed solid state laser is the Erbium laser operating near 3 .mu.m which is either efficiently flashlamp or laser diode excited. The problem which continues to seriously limit the clinical utility of erbium lasers is the lack of a suitable delivery system. Some delivery systems that have been used include articulated arms, hollow waveguides, both single and polycrystalline fibers, and silver halide and metal fluoride fibers. Articulated arms and waveguides are expensive, large, cumbersome, and not easily sterilized. Crystalline fibers, like sapphire, are expensive, and are about 10 times as brittle as quartz fibers of the same diameter. Silver halide fibers are soft, slightly water soluble, and have a poor shelf life. The metal fluoride fibers are the most developed of the 3 .mu.m fiber optics. However, metal fluoride fibers, although tolerable for telecommunications, are unsuitable for many medical applications. In particular, zirconium and aluminum fluoride fibers are brittle, not very flexible, suffer from poor thermal properties, and are toxic and water soluble. Although a sapphire window has been used to isolate the fluoride fiber from tissue, this is not completely reliable and necessarily increases the size and cost of the delivery system.
A tremendous research effort driven by the needs of the telecommunication industry has been directed at developing ultra-low loss fibers suitable for long-haul applications. The emphasis has almost exclusively shifted towards heavy-metal fluoride glasses, based primarily on their intrinsically low losses, wide infrared window, and the relative ease of removing extrinsic impurities which absorb at wavelengths desirable for telecommunications. Their unsuitability for medical laser applications is readily seen in Table A, which compares the fiber requirements for medicine and telecommunications.
TABLE A ______________________________________ Comparison of fiber optic requirements for the medical and telecommunications industries. Requirement Medical Telecommunications ______________________________________ Loss 1 dB/m 1 .times. 10.sup.-4 dB/m Dispersion Irrelevant Must be zero Flexibility Excellent Good Toxicity Must be zero Irrelevant Water solubility Very low Low Strength Very high High Peak power &gt;10.sup.5 W/cm.sup.2 10 W/cm.sup.2 Shelf life 1-5 years 50-100 years Sterilizable Vital Irrelevant Length 1-3 m &gt;10 km ______________________________________
Within the art of fiber optic glass compositions are found germanate glasses for mid-infrared window applications and for other optical elements. A large number of these germanate glasses contain one or more components, such as PbO, BaO, TeO.sub.2, Sb.sub.2 O.sub.3, or As.sub.2 O.sub.3, that render them unsuitable for bio-interactive applications because of their toxicity. Other components added for melt stability or chemical durability shift the multi-phonon edge to shorter wavelengths, which limit the usable length and wavelength of fiber elements.
The present invention provides novel compositions of germanate glass optical fibers that the present state of the art is lacking in several essential areas: non-toxicity, low intrinsic loss, high laser damage threshold, and chemical, thermal, and mechanical stability. These germanate glasses are suitable for laser delivery from 2.5 to 3.0 .mu.m.